For instance, normally open valves, which apply a spring force to a valve element in a valve opening direction, and normally closed valves, which apply a spring force in a valve closing direction, are known as on/off valves which open and close a flow passage. These types of valves are identical in operating principle in which the valve is closed or opened by applying air pressure in a direction against the spring force to the piston connected to the valve element.
As piston actuators for such types of on/off valves, multistage piston actuators in which cylinders each accommodating a piston body in a slidable manner are stacked in stages have been proposed Japanese Unexamined Patent Publication No. 2000-283328, Japanese Patent Publication No. 3,300,686, and Japanese Unexamined Patent Publication No. 2009-2524.